DIFFUSION. 641 



The average velocity of the ions will therefore increase, till the resistance 

 they meet with is equal to the force which urges them forward, and they will 

 thus acquire a definite velocity proportional to the electric force at the point, 

 but depending also on the nature of the liquid. 



If the resistance of the liquid to the passage of the ion is the same for 

 different strengths of solution, the velocity of the ion will be the same for 

 different strengths, but the quantity of it, and therefore the quantity of elec- 

 tricity which passes in a given time, will be proportional to the strength of 

 the solution. 



Now, Kohlrauseh has determined the conductivity of the solutions of many 

 electrolytes in water, and he finds that for very weak solutions the conductivity 

 is proportional to the strength. When the solution is strong the liquid through 

 which the ions struggle can no longer be considered sensibly the same as pure 

 water, and consequently this proportionality does not hold good for strong 

 solutions. 



Kohlrauseh has determined the actual velocity in centimetres per second 

 of various ions in weak solutions under an electro-motive force of unit value. 

 From these velocities he has calculated the conductivities of weak solutions of 

 electrolytes different from those of which he made use in calculating the velocity 

 of the ions, and he finds the results consistent with direct experiments on 

 those electrolytes. 



It is manifest that we have here important information as to the resist- 

 ance which the ion meets with in travelling through the liquid. It is not 

 easy, however, to make a numerical comparison between this resistance and any 

 results of ordinary diffusion, for, in the first place, we cannot make experiments 

 on the diffusion of ions. Many electrolytes, indeed, are decomposed by the 

 current into components, one or both of which are capable of diffusion, but 

 these components, when once separated out of the electrolyte, are no longer 

 ions they are no longer acted on by electric force, or charged with definite 

 quantities of electricity. Some of them, as the metals, are insoluble, and there- 

 fore incapable of diffusion ; others, like the gases, though soluble in the liquid 

 electrolyte, are not, when in solution, acted on by the current. 



Besides this, if we accept the theory of electrolysis proposed by Clausius, 

 the molecules acted on by the electro-motive force are not the whole of the 

 molecules which form the constituents of the electrolyte, but only those which 

 at a given instant are in a state of dissociation from molecules of the other 



VOL. II. 81 



